The present invention relates to a method for selectively removing portions of an abradable coating from a substrate using a mask or stencil and a water jet, or an abrasive water jet to create a pattern of raised ridges on the abradable coating of the substrate. In typical applications of the present invention, the abradable coating may be a thermal barrier coating (TBC) bonded over a bond coat, or it may be a more abradable coating applied over the TBC, such as a TBC having a filler. A typical bond coat applied to turbine components is known in the trade as a MCrAIY coating.
Materials for gas turbine combustion components, such as liners, shrouds, blades, and the like, have reached their limits relative to heat in the turbine which may exceed the melting point of the components. Two methods are currently used to increase component life in the turbine. The first method is to add holes to the component so that air or other cooling gas can exit the holes and create a film of air across the surface which helps keep it cool. The second method is to add a coating, such as a TBC coating, to the surface of the part. The present invention relates to turbine components or other substrates that have a coating added using the second method. By way of example, the shroud of a turbine usually is in the form of a continuous ring or a series of panels sequentially arranged in a cylindrical pattern to form an enclosure for a rotating turbine rotor having radially extending turbine blades. Somewhat recently, an abradable coating has been added to the surface of the TBC on a turbine shroud to allow a better seal between the blade tips and housing. Upon initial rotation, the rotating blades on the turbine rotor actually cut into the abradable coating, creating a better seal which improves compression in the turbine. There are a variety of abradable materials that may be used depending on the particular application, such as, for example, a TBC coating having a polyester filler that makes the coating more abradable, nickel graphite and AlSi-polyester. However, the abradable coating may be formed of a variety of other similar and known materials, depending on the application of the present invention.
Included in the abradable coating is a pattern of raised ridges that project outwardly from the surface of the shroud. Currently, these ridges are formed using a thermal spray process and a mask or stencil. The mask is a flat piece of metal with a pattern of openings cut into it. The abradable coating is sprayed through openings in the mask onto the shroud. The openings in the mask allow for the abradable coating to pass through the mask and onto the surface of the shroud, creating the pattern of raised ridges.
Unfortunately, the abradable coating builds up in the openings in the mask and quickly begins to reduce the amount of coating which is deposited onto the shroud. Because the mask is repeatedly clogged, the mask must be changed frequently, causing interruption in the thermal spray process. These interruptions may result in the coating being formed as a number of stacked layers instead of the preferred single, uniform layer, and in some cases requires a total rework of the component. This increases the cycle time for the process, lowers the quality with the creation of varying mask openings due to coating buildup, decreases coating bond due to the interruption of the thermal spray process to clean the mask, decreases coating bond due to the addition of lubrication on the mask to reduce coating buildup, and/or significantly degrades the coating integrity and product life.
Accordingly, a need exists for a method of creating the ridges on the substrate that avoids the repetitive, labor-intensive process that is created by using the current thermal spray process and mask.